Lightning is a well known natural hazard. Every year in the United States, lightning kills approximately one hundred people and injures on the order of a thousand. Approximately 50 million cloud-to-ground lightning discharges occur each year in the U.S. Damage to equipment and disruption of commercial and industrial operations is measured in billions of dollars. For these reasons, and for many safety-related and equipment protection purposes, it is desirable to provide objective information about impending electrical storms, active thunderstorms and expiring thunderstorms.
Instruments known as electric-field meters are currently used to measure atmospheric electric fields at the surface of the earth. Almost all such electric-field meters are based on a design originally developed by C. T. R. Wilson in the early part of the 20th Century. One of the prior art electric-field meters developed by Wilson used a flat circular metal plate mounted flush with, but well insulated from the ground. The flat plate was connected to a gold-leaf electrometer. To make a measurement of electric field with this instrument, a grounded cover was placed over the sensor plate (thus shielding the sensor plate from the ambient electric field) and a zero for the electrometer was determined. Then the cover was removed, allowing charge to be induced on the plate and causing a deflection of the electrometer leaves. By means of a calibrated variable capacitor and a power supply, Wilson was able to null out the induced charge and thereby determine the electric field at the sensor plate when it was uncovered. All mechanized electric-field meters that followed have been, essentially and simply, variations with varying degrees of automation of the basic concepts employed by Wilson.
Mechanized electrical field meters have been employed for atmospheric research and thunderstorm warning for about seventy years. Mechanized field meters have been used as stand-alone instruments and in networks in which multiple individual sensors are installed some distance apart on the surface of the Earth to give measurements of electric fields over a wide area.
Multiple field meters in a network have been employed at the NASA Kennedy Space Center for more than 20 years as one component of a decision support system to inform official judgement as to propriety of fueling operations, launch, etc. Single field meters are employed at high-risk installations such as armament caches, etc. The cost of commercial field meters currently available is high, they have great electrical power requirements, and they usually need frequent preventive and periodic maintenance. These disadvantages preclude widespread application of commercially available field meters.
More specifically, the prior art electric-field meters suffer from at least three problems which make their wide spread use generally too costly. These three factors are relatively high power consumption, difficult calibration procedure, and stringent requirements for frequent maintenance. For example, on all high input impedance electric-field meters it is necessary to clean the insulators and/or the electrodes of the insulated sensing electrode assembly periodically. Cleaning is necessary because when the insulators become covered with films of dust, moisture and salt spray, conductive paths can form, defeating the purpose of the insulators. Over time, the sensing electrode assembly also becomes covered with a film of dust and salt spray. In the prior art, the cleaning operation is difficult because the prior art electric-field meters require extensive and complex disassembly of the instrument to remove electrodes and thereby clean insulators and electrodes. The disassembly of the prior art electric-field meters for electrode and insulator cleaning thus requires a highly skilled technician adding considerably to the on-going expenses associated with the electric-field meter.
Commercial field meters typically consume tens to hundreds of watts of electrical power. Such high power consumption precludes or discourages application of commercial field meters on most of the existing remote, solar-powered weather stations where electrical power is severely limited.
Commercial field meters, when mounted for practical use in elevated configurations, e.g., above ground, on top of buildings, on weather station masts or poles, for which the electric field enhancement factor is unknown must be properly adjusted to compensate for the mechanically increased gain due to the mounting. Typically this correction is performed by changing the value of a resistor or by adjusting a variable resistor inside the instrument to effect a reduction of the electrical gain of the instrument by the same factor that the gain is enhanced by the mounting arrangement. This gain adjustment process typically involves disassembly of the instrument to gain access to electronic components. This process also typically involves a skilled technician and involves risks of opening and improperly closing sealed enclosures in the field.
Prior-art field meters suffer from two types of uncorrected errors that change with time, temperature, humidity and atmospheric pollutants. Typical instruments that predate the present invention have a zero-signal output (defined as the output value of the field meter with an imposed electric field of zero) that is typically set during manufacture but which subsequently changes in an unknown way with use and time. Because valuable information about atmospheric electrical conditions can be obtained around zero and at the zero-crossing, i.e., when the electric field reverses polarity, there is a significant advantage in having a zero-signal reading that is known with confidence throughout the operating life of the instrument.
Prior-art field meters also suffer from variations in leakage current at the charge-amplifier input due to conduction across insulators associated with the sense electrode and the circuitry used for charge measurement. For prior-art field meters at the place and time of manufacture, the average leakage current at the charge-amplifier input is typically negligible but it invariably increases over time and with changes in atmospheric conditions. The average leakage current in prior-art field meters is an unknown variable that can degrade an instrument to a state of improper operation without warning. Uncorrected increases in average leakage currents tend to reduce the magnitude of the measured electric field, possibly leading to improper assessment of atmospheric electrical threats.
Field meters that suffer from unknown and uncorrected zero offsets and average leakage currents do not always provide information of high quality over long periods of use and such field meters typically require labor intensive testing, adjusting and cleaning at times that have to be determined empirically. Here we teach methods for making field meters that measure and correct zero-signal offset errors and errors due to leakage current at the charge-amplifier input as part of each measurement cycle so that every measurement made and reported is of high quality.
Thus, a need exists in the art for an electric-field meter with low operating power requirements, ease of installation and field calibration, minimal on-going maintenance expenses, and continuous and automatic error detection and correction. It is to such an improved electric-field meter that the present invention is directed.